The use of a coil box in hot-rolling mills is known. The technology of the coil box is such that it can only work with a certain minimum weight of the strip. The minimum weight depends mainly from the strength of the material, the thickness of the strip, and the construction of the coil box. If the weight is below the minimum, there is insufficient friction between the unwinding rolls and the wound-up strip, the coil cannot be unwound. For this reason a strip coil box cannot be used below the minimum weight.
In a coil box the upstream station serves for winding up the coil. Unwinding is started with the coil in the upstream winding station. During unwinding, the coil is transferred to a downstream station. As long as the coil is being unwound in the upstream station, no more strip can be fed into the coil box; there must be some minimum pause in the strip-feeding operation. The minimum pause is dependent of the construction and operation of the coil box.
The residence time in the upstream station is dependent on the speed the material is fed in and pulled out. The maximum speeds are in turn a function of what the equipment can handle upstream and downstream of the coil box and from the production program, and can only be varied within narrow limits. The overall strip production rate is a function therefore of the minimum pause and the maximum material speed so that a coil box can only be used in a hot-strip mill under certain limits.
In the known coil boxes the transfer from the winding to the unwinding station is done by a mechanism. It can be a mandrel engaged in the coil or according to U.S. Pat. No. 5,987,955 by a method with a scale-like tipping action. Even other mechanisms are known as for example rollers on arms that push against the outside surface of the coil. This type of transfer is known as “active transfer” and is used for moving very big coils at very high speeds.
With a mandrelless transfer there is another method of transferring the coil from the winding to the winding station by pulling on the strip of the coil to be unwound from the downstream side. This pulling of the strip is typically done by the upstream equipment, but can also be done partially or fully by another device. In German 1,038,857 a procedure is described that prevents the coil from bumping against fixed abutments and being damaged. This type of transfer is hereinafter described as “passive transfer.” Coils below a certain minimal weight cannot be shifted by passive transfer unless they are held down. The residence time in the upstream station is dependent on the coil weight and can be shorter or longer than when active transfer is employed.
With the known methods using active transfer it is not possible to effect, permit, or even force a passive transfer during an active transfer. In above-referenced U.S. Pat. No. 5,987,955 the scale-like tipping action creates, as a result of the shifting of the unwinding station upstream in the strip-travel direction, gaps in the roller path into which during an active transfer a passively shifting coil drops, with possible damage to the coil and the coil box. If the gap is closed by rollers shifted into place, these rollers must be shifted out of the way during movement of the other is rollers; otherwise there is once again a gap in the roller path.
In most applications a coil box produces a substantially higher and more uniform temperature in the strip. In spite of this the use of a coil box does not produce anything resembling an ideal temperature profile in the strip. The strip inside the coil box loses heat by radiation and contact with the rollers, in particularly at its outer turns.
Different temperatures and a nonuniform temperature profile over the length of the strip are disadvantageous. With inadequate or no strip-thickness control the temperature differences lead to different rolling pressures in the finishing train and to variances of strip thickness along the length of the strip. Uniformity of thickness is however one of the most important quality features in strip finishing.